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Circuit Theory
1740A/1750A/1760 – Series Service Manual
3–37
Diagram 23 XROM
This is a special circuit board that may be present in some 1740A/1750A–Series
instruments. It is not used in the 1760–Series instruments. Where it is
employed, it is physically located next to the CRT.
U5 is a parallel in/serial out shift register. It is used for two purposes, both of
which relate primarily to the assembly and original testing of the circuit board.
The four resistors, R2 – R5, can be used to output a binary code to the processor
for circuit board identification. Their value can be either 0 or 10 k
.
Switch S1 is used as a troubleshooting aid by setting 4 bits either high or low.
U7 is a logic array that decodes Microprocessor outputs. It uses buffered address
0 (BA0) as a clock. Its outputs enable the data and control read and write for the
Flash EPROM.
System level code is loaded into RAM (for reading by the Microprocessor) from
the Flash EPROM, where it is stored. Unless VPP is high (for programming
purposes) the Flash EPROM, U1, U2, U3, & U4, function as a 256k X 8 Read
Only Memory (ROM). (Write instructions are ignored.) U1 & U4 store the
lower 8 bits and U2 and U3 the upper 8 bits. It is read out when FLASH and RD
LO and RD HI are pulled low.
Diagnostics and Circuit
Board Identification
Flash EPROM
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Содержание 1740A Series
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